Using Kid temporarily to charge 24 volt bank from 48 volt bank

[openplanet]

Stupid question, because I think the answer is obvious. But I'd rather sound stupid than be wrong!

I have a large 48 volt 370ah fla bank, charged from a 3.6kW array.  Need to keep a small (80ah), 24 volt agm bank charged.  Can't buy the panels yet, but planning to buy a Kid and use it to charge the 24 volt bank off the 48 volt bank until I get the panels. (The final goal is for the 24 volt system to be fully independent of the larger one.)
Any issue with this?

THanks.

[Highflyer]

Using a Kid to charge one battery bank from another works well.  Consider limiting the charging amps into the 24volt bank from the 48volt bank to around 10 amps due to the small size of your 24 volt bank.  Remember the Kid will run at the 30 amp limit into the 24 volt system unless you limit the charging amps limit.

[openplanet]

Thanks Brian.
Would you say that the most efficient way of accomplishing this is some kind of PWM circuit?

[ClassicCrazy]

I used my Kid for  long time to charge 12v batteries from my 24v pack - worked well .  Once it goes through absorb to float it is going to stay at float . So every once in awhile I would force it into Absorb cycle again. I used Solar but there are other charge modes you can use - forget what the other one is called where you can change some of the parameters but I never used it because I never quite understood it. But Bob and others have a detailed explanation on the forums here somewhere.

And as Highflyers said  - I limited the output current to second battery .

Larry

[Highflyer]

OP,
Wire the 24 volt bank into the battery leads. Then wire the 48 volt bank to the input leads, but first make sure the solar input is OFF. After you are wired up, turn the solar input on and the 48 volt bank will look like solar power to the Kid and it will use that power to charge the 24 volt system.  Remember, limit the amps. 

Look at page 13 of the Kid manual to see the back side of the Kid opened.  In this mode, the Kid will use MPPT to charge the 24 volt bank.  And did I say limit the amps yet?

Hope this helps. 

[mat]

Hi,
As previous answers suggest, it's been succesfully done by others.
I was not so lucky and fried my Kid in 24h trying to charge a 12v fla from 48v fla bank  and yes, i limited amps...
When Midnite repaired it, they said it's not a good idea...

So, at your own risks...

Mat

[CDN-VT]

One would think to do this 48Vdc to 24Vdc through the KID , I'd pick hydro since you should have a constant 48-55Vdc feed but never a night time , still 48Vdc . Then the Kid would or should pick a 24 hr period to reset & record the power it has transferred (stole ) so you can monitor both the 48 harvest & the 24 transfer with a WBjr & a shunt .

I remove all fuses on wire up on the KID & the battery fuse is installed first and should ask you what is the correct voltage (and I would pick hydro over solar) of the system after all the LEDS stop the flashing . Here you can NOW program all of the settings and limit , set charge rates etc.

This takes quite a bit of time to get all the stuff in correctly & check. After checking and your sure , I would install solar fuse of 20 amps (down grade it from the 30 or even 15 since your limiting the transfer) after all that , go back into menu , hydro & then turn on ,a click will be heard & check the screens .

Remember BIG ONE !! The 48Vdc & the 24Vdc DO NOT share the common neg buss . So each pos & neg from each voltage system will have it's own terminal ..

VT

[Highflyer]

VT,
The first question I would want the answer to before using the hydro setting would be whether or not the 48 volts system stays fully charged or not.  If not, wouldn't the sweep of a solar setting be more appropriate?  As the 48 volt system charges and discharges, the sweep would find that voltage for the charge.

I must admit, I don't know the answer to this one and look forward to finding out which is better.

[ClassicCrazy]

Hi,
As previous answers suggest, it's been succesfully done by others.
I was not so lucky and fried my Kid in 24h trying to charge a 12v fla from 48v fla bank  and yes, i limited amps...
When Midnite repaired it, they said it's not a good idea...

So, at your own risks...

Mat

What did you limit the amps to Mat ?  What fried on your Kid ? Mosfets ? I wonder if overheating is what did it in .

Larry

[CDN-VT]

Use the Classic 48Vdc on the AUX to turn on a relay to send power to the kid .
That way you will not drop the 48Vdc bank to death to help a 24Vdc bank .

I see this in my mind , but maybe not finger typed out well

[mat]

To Larry,
As my memory isn't too good, here's the post i did when that happened...


http://midniteftp.com/forum/index.php?topic=3379.0

[tecnodave]

To all,

I have written a detailed reasoning as to why this is not a good idea. I'm going to post it in a new category in hopes that it will be more widely read. The category will be  "How to charge a lower voltage battery from a higher voltage battery"  I've done much research on this subject

Hope it will help

David

[Vic]

Remember BIG ONE !! The 48Vdc & the 24Vdc DO NOT share the common neg buss . So each pos & neg from each voltage system will have it's own terminal ..
VT

Completely agree with VT.   If the two battery banks share a COMMON NEGATIVE,   the KID's Shunt that measures input current is SHORTED.   So,  one would expect that the KID would keep trying to increase its output current,   because it is getting a false low current indication.

Things would be less-bad,   if the battery being charged from the KID did NOT share a common negative with the higher voltage battery connected to the KID's input.

Agree also,   that  trying to use the KID's Load terminals for the lower voltage battery is a very bad idea.

FWIW,   Vic

[tecnodave]

Vic,

Thanks for pointing that one out, I do know that, just forgot to mention that in my last post in "How to charge lower voltage batteries from higher voltage batteries"

TD

[Highflyer]

OP,
It looks like I got lucky charging from batteries using a Kid.  I have not had a problem, but you might.  What I do know is that these guys know their stuff and I am going to listen them from now on.

Sounds like the chargers TD is talking about will do the job well.  Or just wait until you get the panels needed for your other setup.